1. Field of the Invention
The present invention relates to an arc tube for a discharge lamp having a closed chamber, into which metal halides containing at least Na and Sc are sealed together with a rare gas, in which electrodes are provided to oppose to each other, and an internal volume of which is 50 μl or less.
2. Description of the Related Art
FIG. 14 shows a conventional discharge lamp. The discharge lamp has such a structure that a front end portion of a quartz-glass arc tube 5 is supported with one lead support 2 protruded forward from an insulating base 1, a rear end portion of the arc tube 5 is supported with a concave portion 1a of the base 1, and the arc tube 5 is sustained at a portion near its rear end with a metal supporting member 4 fixed to a front surface of the insulating base 1. A front end-side lead wire 8 led from the arc tube 5 is fixed to the lead support 2 by the welding, while a rear end-side lead wire 8 is passed through a bottom wall 1b constituting the concave portion 1a of the base 1 and secured to a terminal 3 provided to the bottom wall 1b by the welding. A symbol G is a cylindrical ultraviolet shielding globe made of the glass to cut off an ultraviolet component in a bandwidth that is harmful to a human body from the light that is emitted from the arc tube 5. This ultraviolet shielding globe G is deposited integrally to the arc tube 5.
Then, the arc tube 5 has such a structure that a closed glass globe 5a in which electrodes 6, 6 are provided between a pair of front and rear pinch sealed portions 5b, 5b to oppose to each other and into which luminous substances i.e., Na halides, Sc halides or Hg, are sealed together with a starting rare gas is formed. A molybdenum foil 7 for connecting the electrode 6 protruded into the closed glass globe 5a and the lead wire 8 led from the pinch sealed portion 5b is sealed in the pinch sealed portion 5b, and thus an air tightness in the pinch sealed portion 5b is maintained.
In this case, this Hg sealed in the closed glass globe 5a is a very useful buffer substance to relieve the damage of the electrode by maintaining a predetermined tube voltage and reducing an amount of collision of the electron to the electrode 6. However, such Hg is an environmentally hazardous material. For this reason, recently the development of the so-called mercury-free arc tube into which Hg acting as the environmentally hazardous material is not sealed is accelerated.
Then, in Japanese Patent Unexamined Publication No. JP-A-2002-93369, it was proposed that a second metal (at least one type or plural types of Mg, Fe, Co, Cr, Zn, Ni, Mn, Al, Sb, Be, Re, Ga, Ti, Zr, and Hf), which is hard to emit the light in the visible range rather than a first metal (Na or Sc) that is popular as the luminous substance, is sealed instead of Hg, and thus such Hg should not be sealed at all or a small amount of Hg should be sealed if any.
Then, in the course of the development of the mercury-free arc tube, the inventors trially manufactured the embodiments (referred to as “Literature Embodiments” hereinafter) that are disclosed in the Japanese Patent Unexamined Publication No. JP-A-2002-93369, and then examined a tube voltage, a luminous flux, and a luminous flux build-up of respective trial arc tubes within 0 time in practical use (referred to as “initial characteristics” hereinafter). At that time, none of them could satisfy all the initial characteristics, as shown in FIG. 15. In FIG. 15, Literature Embodiments 2, 3 provide the mercury-containing arc tube in which a minute quantity (1 mg) of Hg is sealed, respectively. Also, Literature Embodiments 1, 4 to 6 provide the mercury-free arc tube in which other metal halides are sealed instead of Hg respectively, wherein a ScI3 sealed density is set to 2.8 mg/ml in Literature Embodiments 1 to 5 and a ScI3 sealed density is set to 3.4 mg/ml in Literature Embodiment 6.
The inventors concluded that a cause of unsatisfactory initial characteristics lies in the low sealing pressure (0.1 or 0.5 MPa) of the rare gas (Xe-gas). Then, the inventors trially manufactured the mercury-containing arc tubes in which a minute amount (0.72 mg) of Hg is sealed, a ScI3 sealing density is set to 3.28 mg/ml, and a Xe-gas sealing pressure is set differently respectively, as shown in FIGS. 2 and 3. Then, the initial characteristics are performed evaluation test. At this time, it was checked that, as shown in FIGS. 3 and 4, the Xe-gas sealing pressure of 0.6 MPa or more should be desired to satisfy the initial characteristics i.e., tube voltage, luminous flux and luminous flux build-up. In other words, it is estimated that the initial characteristics could be improved because a pressure in the closed glass globe is high when the tube is turned ON.
However, such a new problem has arisen that a flicker phenomenon of a light during its lightened state of the arc tube is generated. Hereinafter, this phenomenon is referred to as the “flicker”.
Reaction formulas of the flicker occurring mechanism are given as shown in below.4ScI3+3SiO2→2Sc2O3+3SiI4  (1)nW+SiI4→SiWn+2I2  (2)4ScI3+3ThO2→2Sc2O3+3ThI4  (3)
Such flicker occurring mechanism will be explained as follows.
That is, the quartz glass (SiO2) constituting a tube wall of the arc tube reacted with ScI3, as given by Formula (1), to generate a devitrification phenomenon. Then, SiI4 generated at this time reacted with the tungsten electrode, as given by Formula (2), to generate a low-melting alloy (SiWn). Also, in the thoria-doped tungsten (which is also called thoriated tungsten) electrode, thoria (ThO2) disappeared as given by Formula (3). Then an inter-electrode distance which is defined between the electrodes was expanded by the deformation or damage of the electrode, and also a re-ignition voltage was increased, so that a ballast uncontrollable state was brought about to cause the flicker. As a result, it is estimated that the reactions to cause the flicker are accelerated because the pressure in the arc tube (closed glass globe) is high when the tube is turned ON.
Here, the inventors concluded that ScI3 is concerned largely with generation of the devitrification phenomenon and disappearance of the thoria, which result in the deformation of the electrode, and that any correlation exists between a ScI3 sealing density and a flicker occurring rate.
Then, as shown in FIGS. 5 to 9, the inventors trially manufactured the arc tubes having different specifications while differentiating the internal volume of the closed chamber, the Xe gas sealing pressure, the ScI3 sealing density, mercury contained or mercury not contained (the metal halide such as In halide, or the like is sealed in place of Hg as the sealed buffer substance), etc., and then examined whether or not the flicker occurred. At this time, it was derived from the data shown in FIGS. 5 to 9 that correlations shown in FIGS. 10 and 11 reside between the ScI3 sealing density and the flicker occurring rate. The flicker occurring rate is increased sharply if the ScI3 sealing density exceeds 4.7 mg/ml, and also deformation and damage of the top end portion of the electrode grow worse, which is shown that the inter-electrode distance is extended, as the ScI3 sealing density becomes high and the flicker occurring rate becomes high.
That is, it was found that the ScI3 sealing density should be lowered in order to lower the flicker occurring rate, and no flicker occurs at all if the ScI3 sealing density is lowered rather than 4.7 mg/ml.
Also, it was confirmed that the correlation shown in FIG. 12 is present between the ScI3 sealing density and the luminous efficiency (lumen/W) and also a lower limit of the ScI3 sealing density should be set to 1.25 mg/ml because the luminous efficiency of at least 75 lumen/W is needed as the car lamp for example, a headlamp.
In this manner, in order to satisfy the initial characteristics i.e., tube voltage, luminous flux, and luminous flux build-up, of the arc tube for the discharge lamp, it is desired that the Xe gas sealing pressure should be set to 0.6 MPa or more, as shown in FIGS. 3 and 4. Then, it was confirmed that the flicker whose occurrence is worried when the Xe gas sealing pressure is set high i.e., 0.6 MPa or more, can be suppressed by setting the ScI3 sealing density to 4.7 mg/ml or less, as shown in FIGS. 10 and 11, and the luminous efficiency i.e., 75 lumen/W or more, required as the car lamp can be assured (see FIG. 12) by setting the ScI3 sealing density to 1.25 mg/ml or more. As a result, these findings lead the inventors to propose the present invention.